Electro-optic systems in which an electrophoretic-like or dipolar material is dispersed throughout a liquid crystal to reduce the turn-off time

ABSTRACT

A system for transforming an optically negative liquid crystalline mesophase composition to an optically positive liquid crystalline mesophase composition by an applied electrical field, and imaging compositions which facilitate the relaxation of the transformed optically positive composition into the initial optically negative state. The invention also encompasses imaging systems wherein this electrical field-induced transition images a liquid crystalline imaging composition.

Unite States Patent Wysocki [54] ELECTRO-OPTIC SYSTEMS IN WHICH ANELECTROPHORETIC-LIKE OR DIPOLAR MATERIAL IS DISPERSED THROUGHOUT ALIQUID CRYSTAL TO REDUCE THE TURN-OFF TIME [72] inventor: Joseph J.Wysocki, Webster, NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: Jan. 6, 1971 [21] Appl. No.: 104,366

[52] 11.5. C1. ..350/150, 252/408, 350/160 [51] Int. Cl. ..G02f 1/16[58] Field of Search ..252/300, 408; 350/150, 160

[56] References Cited UNITED STATES PATENTS 3,519,330 7/1970 Heilmeier..350/160 R [451 Oct. 10, 1972 3,551,026 12/1970 Heilmeier ..350/1503,575,492 4/1971 Nester et a1 ..350/160 R 3 ,575,493 4/1971 Heilmeier..350/160 R Primary Examiner-Ronald L. Wibert Assistant Examiner-EdwardS. Bauer Attorney-James J. Ralabate, David C. Petre and Roger W.Parkhurst [5 7] ABSTRACT A system for transforming an optically negativeliquid crystalline mesophase composition to an optically positive liquidcrystalline mesophase composition by an applied electrical field, andimaging compositions which facilitate the relaxation of the transformedoptically positive composition into the initial optically negativestate. The invention also encompasses imag ing systems wherein thiselectrical field-induced transition images a liquid crystalline imagingcomposition.

20 Claims, 5 Drawing Figures PATENTEU B I973 3.697.150

FIG. 4

INVENTOR. l5 JOSEPH J. WYSOCKI A 7' TOR/V5 Y ELECTRO-OPTIC SYSTEMS INWHICH AN ELECTROPHORETIC-LIKE OR BIPOLAR MATERIAL IS DISPERSEDTHROUGHOUT A LIQUID CRYSTAL TO REDUCE THE TURN-OFF TIME BACKGROUND OFTHE INVENTION This invention relates to electro-optic systems, and morespecifically to electro-optic systems including liquid crystallinecompositions which facilitate the relaxation or switching time of suchelectro-optic systems. Furthermore, this invention also relates toimaging systems wherein the imaging member comprises a liquidcrystalline composition, and to compositions to facilitate therelaxation or switching time of such imaging systems.

There has recently been widespread interest in the discovery of moreuseful applications for the class of materials known as liquid crystals.The name liquid craystals has become generic to liquid crystallinematerials which exhibit dual physical characteristics, some of which aretypically associated with liquids, and others which are typically uniqueto crystalline solids. Liquid crystals exhibit mechanicalcharacteristics such as viscosities, which are ordinarily associatedwith liquids. The optical characteristics of liquid crystals are moresimilar to those characteristics ordinarily unique to crystallinesolids. In liquids or fluids, the molecules are typically randomlydistributed and oriented throughout the mass of the material.Conversely, in crystalline solids molecules are generally rigidlyoriented and arranged in a specific crystalline lattice structure.Liquid crystals resemble solid crystals in that the molecules of liquidcrystalline substances are regularly oriented in a fashion analogous to,but less extensive than, the molecular orientation and structure in acrystalline solid. Many substances have been found to exhibit liquidcrystalline characteristics in a relatively narrow temperature range;however below that temperature range the substances typically appear ascrystalline solids, and above that temperature range they typicallyappear as liquids.

Liquid crystals are known to appear in three different mesomorphicforms: the smectic, nematic, and cholesteric. In the nematic structure,the molecular organization is characterized by the approximatelyparallel orientation of the major axes of the individual molecules withrespect to one another. In the cholesteric structure, the molecules arebelieved to be arranged in a series of distinguishable layers, andwithin any such layer, the molecules are believed to be arranged withtheir major axes approximately parallel to each other. The major axes ofthe molecules in the cholesteric structure are believed to be parallelto the planes of the layers, and the molecular layers are thin.Additionally, when compared to a hypothetical straight line axis passingthrough a cholesteric liquid crystalline material perpendicular to theplanes of molecules within the material, it is seen that the paralleldirection of the molecular axes of the adjacent molecules within a givenlayer is angularly displaced with respect to the straight line axis,thereby tracing out a helical path around the straight line axis. It isbelieved that this helically displaced layered structure is caused bythe shape of the molecules within the thin layers. The cholestericstructure originally derived its name from the fact that materialsexhibiting the cholesteric liquid crystalline mesophase structuretypically were molecules which were derivatives of cholesterol or whichwere shaped very similarly to molecules of cholesterol.

Liquid crystals have been found to be sensitive or responsive to variousstimuli including electrical fields, as disclosed, for example, incopending application Ser. No. 646,532, filed June 16, 1967; copendingapplication Ser. No. 4,644, filed Jan. 21, 1970; and French Pat. No.1,484,584. Most recently, electro-optic systems and imaging systemswherein the imaging member comprises a liquid crystalline material havebeen discovered, and are described in copending applications Ser. No.821,565, filed May 5, 1969, now U.S. Pat. No. 3,652,148 and Ser. No.47,698, filed June 19, 1970.

Various combinations of liquid crystalline compositions with otheradditives are known. For example, Fergason U.S. Pat. No. 3,409,404 showsthe compatibility of cholesteric liquid crystalline materials with oiladditives, and Fergasons article in Applied Optics, Vol. 7, No. 9,September, 1968, p. 1730, shows that dyes may be added to cholestericmaterials. Dreyer U.S. Pat. No. 2,544,659, French U.S. Pat. No.3,440,620, and Goldmacher et al. U.S. Pat. No. 3,499,702 disclosenematic liquid crystalline compositions with additives or guests.However, in new and growing areas of technology such as liquidcrystalline electro-optic and imaging systems, new methods, apparatus,compositions, and articles of manufacture are often discovered for theapplication of the new technology in a new mode. Similarly, further newuses and surprising and advantageous results of uses of such newtechnology are being discovered. The present invention relates to a newsystem, and new and advantageous imaging compositions, for decreasingthe relaxation or switching time in liquid crystalline electro-opticsystems.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide a novel electro-optic system.

It is another object of this invention to provide a novel liquidcrystalline imaging system.

It is another object of this invention to provide novel compositions foruse in liquid crystalline electro-optic systems.

It is another object of this invention to transform an opticallynegative liquid crystalline composition to an optically positive liquidcrystalline composition by an applied electrical field.

It is another object of this invention to transform a cholesteric liquidcrystalline composition to a nematic liquid crystalline composition inan applied electrical field.

It is another object of this invention to provide a new means forreducing the relaxation time, or controlling the return of a transformedliquid crystalline electrooptic .system to its initial or equilibriumstate.

It is another object of this invention to facilitate the relaxation of aliquid crystalline material in a field-induced optically positive stateinto its normal optically negative state without the application ofexternal stimuli.

It is yet another object of this invention to provide a color imagingsystem.

The foregoing objects and others are accomplished in accordance withthis invention by a system for trans forming an optically negativecholesteric liquid crystalline composition to an optically positivenematic liquid crystalline composition by an applied electrical field.This system includes imaging compositions which facilitate therelaxation of the transformed optically positive composition into theinitial optically negative state. The invention also encompasses imagingsystems wherein this electrical field-induced transition images a liquidcrystalline imaging composition.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of theinvention as well as other objects and further features thereof,reference is made to the following detailed disclosure of preferredembodiments of the invention taken in conjunction with the accompanyingdrawings thereof, wherein:

FIG. 1 is a partially schematic, cross-sectional view of an embodimentof a liquid crystalline electro-optic imaging member.

FIG. 1A is a magnified portion of the cross-sectional view of FIG. I.

FIG. 2 is a partially schematic isometric view of an embodiment of aliquid crystalline imaging member wherein the desired image is definedby the shape of the spacing member.

FIG. 3 is a partially schematic isometric view of an embodiment of aliquid crystalline imaging member wherein the desired image is definedby the shape of at least one facing electrode.

FIG. 4 is a partially schematic isometric view of an embodiment whereina liquid crystalline imaging member is viewed between polarizers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a liquidcrystalline, electro-optic cell Ill in partially schematic,cross-sectional view wherein a pair of supporting plates 11 havingconductive coating 12 upon the contact surface thereof, comprise asubstantially parallel pair of electrodes. The supporting plates 11 maybe transparent, and the conductive coatings 12 may also be substantiallytransparent conductive coatings. A cell wherein both electrodes aretransparent is preferred where the cell is to be used with transmittedlight; however, such liquid crystalline electro-optic cells may also beused with reflected light thereby requiring only a single transparentelectrode while the other may be translucent or opaque. The electrodes,which for the purposes of illustration shall be referred to astransparent electrodes, are typically separated by spacing or gasketmember 13 which contains voids which form one or more shallow cups whichmay contain the liquid crystalline film or layer which comprises theactive element in the electro-optic cell. An electrical field is createdbetween the electrodes by an external circuit 15 which typicallycomprises a source of potential 16 which is connected across the twoelectrodes through leads 17. The potential source may be either D.C.,AC, or a combination thereof.

FIG. 1A is a partially schematic magnification of a portion of thecross-sectional view of FIG. 1 showing the support members 11 with theconductive or substantially transparent conductive coatings l2 thereonwith the liquid crystalline film or layer 14 enclosed between theconductive electrodes. In addition, in the present invention the liquidcrystalline composition also comprises or includes the advantageousadditive 14A which is dispersed throughout the liquid crystallineelectro-optic composition matrix.

The liquid crystalline electro-optic cell described in FIG. 1 and FIG.1A may, in various embodiments, be used as an imaging system, forexample as illustrated in FIGS. 2 and 3. In FIG. 2 an embodiment of sucha liquid crystalline electro-optic imaging member is shown with thedesired image configuration defined by the shape of the void areas inthe spacer gasket 13. As before, transparent electrodes 18 are separatedby the spacer 13, but the desired image area 19 comprises the liquidcrystalline film or layer. In this embodiment the entire inner faces ofthe transparent electrodes comprise substantially transparent conductivecoating 12, and the conductive coatings are electrically connected toexternal circuit 15. In operation there is an electrical field acrosstheentire area of the spacer 13, however the electro-optic imaging effectin the liquid crystalline film causes imaging to occur only in the area19 where the liquid crystalline film is present. Again here, dependingupon whether the desired image is to be viewed by transmitted orreflected light, both or only one of the electrodes, respectively, maybe transparent.

In FIG. 3 another preferred embodiment of the liquid crystallineelectro-optic imaging member is shown. Here the desired image is definedby the shape of an electrode, and therefore by the shape of thecorresponding electrical fields produced by the system including thatelectrode. The imaging member here comprises transparent support plates11 separated by spacer gasket 13 having void area 20 filled with liquidcrystalline electro-optic composition, and the area of the .void 20, andits included liquid crystalline imaging composition, comprisessubstantially the entire area of the spacer layer 13. The desired imageis defined by the shape of the substantially transparent conductive coating here shown in imagewise configuration 21, which is affixed to theinner surface of one or both of the transparent support plates 11, andis affixed in the desired image configuration. The embodimentillustrated in FIG. 3 shows only one of the two electrodes in imagewiseconfiguration; however, it will be understood by those skilled in theart that both electrodes could easily be made in a congruently matchedpair to define the same desired image. When the single imagewiseelectrode configuration is used, the second electrode will typicallycomprise transparent plate 11 with substantially transparent conductivecoating 12 upon the entire area of the inner surface of the transparentsupport 11. It is noted that a very thin, or substantially invisibleconductor 22 is typically used in this embodiment to electricallyconnect the electrode in the desired image configuration to externalcircuit 15, which is similarly connected to the conductive coating ofthe opposite electrode. In operation this embodiment typically produceselectrical fields only in areas where there are parallel electrodes,i.e., between the electrode and the desired image configuration, and theopposite electrode, whether or not the second electrode is also in thedesired image configuration. Again here, one of the electrodes may beopaque if it is desired to observe the imaging member by reflected lightrather than transmitted light.

The electro-optic effect of the liquid crystalline imaging members ofthe present invention, which may also be used as a liquid crystallineelectro-optic imaging system, is in part described in copendingapplication Ser. No. 821,565, filed May 5, 1969 and now US. Pat. No.3,652,148, the entire disclosure of which is hereby expresslyincorporated by reference in the present specification. In the systemdescribed in that copending application and in the system of the presentinvention,

cholesteric liquid crystals, a mixture of cholesteric liquid crystallinematerials, or a composition exhibiting cholesteric liquid crystallinecharacteristics is used in an electrode sandwich configurationembodiment such as that described in FIG. 1 so that high strengthelectrical fields across the liquid crystalline composition film causean electrical field-induced phase transition to occur wherein theoptically negative cholesteric liquid crystalline composition istransformed into an optically positive liquid crystalline state. Theelectro-optic cholesteric liquid crystalline compositions suitable foruse in the present invention typically have a transition threshold fieldstrength at or above which the advantageous transition takes place. Thistransition is believed to be the result of the cholesteric liquidcrystal transforming into the nematic liquid crystalline mesophasestructure. This is believed to be a bulk effect which affects the entirecross-section of the transformed portions of the composition layer.Cholesteric liquid crystals, or compositions exhibiting cholestericliquid crystalline characteristics are typically translucent, forexample, like a milky white, opalescent layer, when placed in theunbiased electrode sandwich. When the high strength electrical fieldsare placed across the liquid crystalline film, the field-induced phasetransition is observable because the liquid crystal film becomessubstantially transparent in areas where the field is present.

When viewed between polarizers with transmitted light, for example asillustrated in FIG. 4, the areas in which the field-induced phasetransition has taken place appear dark, while the unchanged,translucent, light scattering and birefringent composition stillexhibiting the cholesteric liquid crystalline characteristics retainsthe light-colored appearance. When such a liquid crystallineelectro-optic cell is observed between polarizers, the light source 24emits light which is planar polarized while passing through polarizer23a, scattered by the translucent cholesteric liquid crystallinecomposition in non-image areas 25 (the same as areas 20 in FIG. 3), andis transmitted by the field-induced nematic liquid crystalline areas 26.A viewer 27 sees the planar polarized light which passes throughpolarizer (or analyzer) 23b which originated from source 24 and wasscattered and passed through the non-imaged portion of the liquidcrystalline electrooptic composition contained between the electrodes 18by spacer gasket 13. It will be appreciated by those skilled in the artthat polarizers 23a and 23b (or polarizer and analyzer combination) aretypically used so that the planes of polarization of the two polarizersare approximately normal to each other. Of course the two polarizers maybe rotated with respect to one another to achieve any desired degree ofcontrast of which the particular system is capable. Although the lightpolarized by polarizer 23a is typically polarized in a plane crossedwith the plane of polarizer 23b, the effect of the cholesteric liquidcrystalline composition between the substantially transparent electrodesis to scatter sufficient amounts of the originally planarpolarized lightto allow some of it to pass through polarizer 2312. However, in theimage areas 26, the effect of polarizers 23, when said polarizers havetheir respective planes of polarization crossed approximately normal toeach other, is to substantially cut off the transmission of lightthrough the polarizer 23a in the transformed image area 26 so that theimage area 26 appears dark, as illustrated in FIG. 4.

Any other means suitable for enhancing the contrast of the imaged areasmay also be used in place of the polarizers. For example various lightfiltering systems, or differential lighting systems may be used. Hence,it is seen that either field or non-field areas in an electrooptic,liquid crystalline imaging sandwich may be used to create the desiredimage, with or without the addition of means for image enhancement.

In the liquid crystal electro-optic cells and imaging members describedherein the electrodes may be of any suitable transparent conductivematerial. Typical suitable transparent, conductive electrodes includeglass or plastic substrates having substantially transparent andcontinuously conductive coatings of conductors such as tin, indiumoxide, aluminum, chromium, tin oxide, or any other suitable conductor.These substantially transparent conductive coatings are typicallyevaporated onto the more insulating, transparent substrate. NESA glass,a tin oxide coated glass manufactured by the Pittsburgh Plate GlassCompany, is a commercially available example of a typical transparent,conductive electrode material. Of course where opaque electrodes areused herein, any suitable electrically conductive material may be used.

The spacer, 13 in FIG. 1, which separates the transparent electrodes andcontains the liquid crystal film between said electrodes, is typicallychemically inert, transparent, not birefringent, substantiallyinsulating and has appropriate dielectric characteristics. When the cellis to be used with polarizers, it is also desireable that the spacermaterial be optically isotropic. Materials suitable for use asinsulating spacers include cellulose acetate, cellulose triacetate,cellulose acetate butyrate, polyurethane elastomers, polyethylene,polypropylene, polyesters, polystyrene, polycarbonates,polyvinylfluoride, polytetrafluorethylene, polyethylene terephthalate,and mixtures thereof.

The liquid crystal imaging film 14 may comprise any suitable cholestericliquid crystal, mixture of cholesteric liquid crystals, or compositionwhich exhibits cholesteric liquid crystalline characteristics. Typicalcholesteric liquid crystals include derivatives from reactions ofcholesterol and inorganic acids; for example, cholesteryl chloride,cholesteryl bromide, cholesteryl iodide, cholesteryl fluoride,cholesteryl nitrate; esters derived from reactions of cholesterol andcarboxylic acids; for example cholesteryl crotonate; cholesterylnonanoate; cholesteryl hexanoate; cholesteryl formate; cholesteryldocosonoate; cholesteryl chloroformate; cholesteryl propionate;cholesteryl acetate; cholesteryl valerate; cholesteryl vacconate;cholesteryl linolate; cholesteryl linolenate; cholesteryl oleate;cholesteryl erucate; cholesteryl butyrate; cholesteryl caprate;cholesteryl laurate; cholesteryl myristate; cholesteryl clupanodonate;ethers of cholesterol such as cholesteryl decyl ether; cholesteryllauryl ether; cholesteryl oleyl ether; cholesteryl dodecyl ether;carbarnates and carbonates of cholesterol such as cholesteryl decylcarbonate; cholesteryl oleyl carbonate; cholesteryl methyl carbonate;cholesteryl ethyl carbonate; cholesteryl butyl carbonate; cholesteryldocosonyl carbonate; cholesteryl cetyl carbonate; cholesteryl heptylcarbarnate; and alkyl amides and aliphatic secondary amines derived from33 -amino- AS-cholestene and mixtures thereof; peptides such ascholesteryl polybenzyl-l-glutamate; derivatives of beta sitosterol suchas sitosteryl chloride; and amyl ester of cyano benzylidene aminocinnamate. The alkyl groups in said compounds are typically saturated orunsaturated fatty acids; or alcohols, having less than about 25 carbonatoms, and unsaturated chains of less than about five double-bondedolefinic groups. Aryl groups in the above compounds typically comprisesimply substituted benzene ring compounds. Any of the above compoundsand mixtures thereof may be suitable for cholesteric liquid crystallinefilms in the advantageous system of the present invention.

Smectic liquid crystalline materials are suitable for use as componentsof the imaging composition in the present invention and such smecticliquid crystal materials include: n-propyl-4-ethoxybiphenyl-4-carboxylate; 5-chloro-6-n-heptyloxy-2-naphthoic acid; lowertemperature mesophases of cholesteryl octanoate, cholesteryl nonanoate,and other open-chain aliphatic esters of cholesterol with chain lengthof seven or greater; cholesteryl oleate; sitosteryl oleate; cholesteryldecanoate; cholesteryl laurate; cholesteryl myristate; cholesterylpalmitate; cholesteryl stearate; 4-n-alkoxy-3-nitrobiphenyl-4-carboxylicacids ethylp-azoxy-cinnamate; ethyl-p-4-ethoxybenzylideneaminocinnamate;ethyl-p-azoxybenzoate; potassium oleate; ammonium oleate;p-n-octyloxy-benzoic acid; the low temperature mesophase of2-p-n-alkoxybenzylideneaminofluorenones with chain length of seven orgreater; the low temperature mesophase of p- (n-heptyl)oxybenzoic acid;anhydrous sodium stearate; thallium l) stearate; mixtures thereof andothers.

Nematic liquid crystalline materials suitable for use as components ofthe imaging composition in the advantageous system of the presentinvention include: pazoxyanisole, pazoxyphenetole, p-butoxybenzoic acid,p-methoxy-cinnamic acid, butyl-p-anisylidene-paminocinnamate,anisylidene paraamino-phenylacetate,p-ethoxy-benzalamino-a-methyl-cinnamic acid, l,4-bis(p-ethoxybenzylidene) cyclohexanone, 4,4'-dihexyloxybenzene,4,4'-diheptyloxybenzene, anisal-p-amino-azo-benzene, anisaldazine,abenzeneazo- (anisal-a-naphthylamine), n,n-nonoxybenzetoluidine; anilsof the generic group (p-n-alkoxybenzylidene-p-n-alkylanilines), such asp-methoxybenzylidene p-n-butylaniline, mixtures of the above and manyothers.

The above lists of materials exhibiting various liquid crystallinephases are not intended to be exhaustive or limiting. The lists disclosea variety of representative materials suitable for use in the imagingcomposition or mixture comprising cholesteric liquid crystallinematerials, which comprises the active imaging element in theadvantageous system of the present invention.

Mixtures of liquid crystals or liquid crystalline compositions can beprepared in organic solvents such as chloroform, petroleum ether,methylethyl ketone and others, which are typically subsequentlyevaporated from the composition after the composition is placed in thedesired electro-optic cell or on a suitable electrode or other suitablesubstrate. Alternatively, the individual liquid crystals of the liquidcrystalline mixture or composition can be prepared by heating the mixedcomponents above the liquid isotropic transition temperature of thecomposition or of its components, thoroughly mixing the liquidcomposition, and placing the liquid composition in the cell or on asuitable electrode or substrate, and allowing the composition to coolinto its cholesteric liquid crystalline mesophase temperature range.Room temperature liquid crystals may be used in their natural condition.It should be noted that in some embodiments, after preparation by theabove means, the layer of liquid crystalline composition in the presentinvention may itself possess sufficient integrity to remain in place inthe electro-optic imaging system without a confining gasket. Forexample, the composition may have a sufficiently high viscosity tomaintain its position on a surface when the surface is orientedvertically or even horizontally with the composition layer on the lowersurface (i.e., the adhesion of the viscous composition is sufficient toover come gravitational force).

The layers of electro-optic composition having cholesteric liquidcrystalline characteristics are typically of thickness not greater thanabout 250 microns, although thicker layers may perform satisfactorily insome embodiments of the inventive system. For preferred imaging results,the composition layers are preferably of thickness in the range betweenabout 1 and about 50 microns. It will be appreciated that thinner layersof the electro'optic composition will require smaller voltages acrossthe layer to produce the desired results in the inventive system.

In the advantageous electro-optic system described in application Ser.No. 821,565 now U.S. Pat. No. 3,652,148, which is similarly used in thepresent invention, the high strength electrical fields which typicallyproduce the desired results are generally in the range between about 10and 10 volts/cm. of thickness of the layer of electro-optic liquidcrystalline composition, and preferred electro-optic and imaging resultsare typically achieved using field strengths on the order of aboutl0--l0 volts/cm.

The turn-on switching time for such electro-optic cells or imagingsystems is typically not greater than about 1 second. Any applied fieldstrength of magnitude greater than the transition threshold fieldstrength of the cholesteric composition is typically sufficient to causethe desired effect. However, it is found that field strengths somewhathigher than the transition threshold field strength typically reduce theturn-on switching or response time of the system.

The electrical field-induced optically negative to optically positivephase transition system of application Ser. No. 821,565, and of thepresent invention, has also been found to be suitable as a transientcolor display when the field strength of the electrical field across theelectro-optic liquid crystalline composition layer is decreased to avalue below the threshold field required to produce the phase transitionin the composition layer. With the field strength below the thresholdfield value, the field-induced transformed portions of the compositionlayer are allowed to resume their original, optically negativecholesteric liquid crystalline state. This re-transformation orrelaxation may occur over a period which may vary from fractions of asecond to minutes or hours, depending upon the specific embodiment ofthe inventive system. Transient electro-optic or electro-optic imagingsystems may make use of all or any portion of this relaxationtransition. It is noteworthy that the transient display characteristicsare observed when the relaxation or retransformation occurs in theabsence of an electrical field. As the initially sharp, field-inducedoptically positive, nematic transformed portions of the compositionlayer relax back into their original, optically negative cholestericstate, the birefringence and optical activities of the liquidcrystalline material changes drastically giving rise to changing colorpatterns which stand out vividly from non-transformed portions of theliquid crystalline composition layer or from other surroundingbackground areas. In many many embodiments this relaxation orretransformation occurs over a sufficient length of time, i.e. at leasta few seconds, to be quite useful as a dynamic color display system.

It will be appreciated that in electro-optic cells or imaging systemsutilizing the electro-optic liquid crystalline phase transition systemof the present invention, and especially where such displays or imagingsystems are to be recycled or reimaged in short time sequences, that itis highly desireable to control, and especially to minimize, the tum-offtime of such systems. The tum-off response time or relaxation time inthe present invention is intended to mean the time which is typicallyrequired for a substantially complete relaxation of the electroopticcomposition from its induced optically positive nematic state back intoits natural optically negative cholesteric state.

Surprisingly, in the system of the present invention it has been foundthat the inclusion of various advantageous additives, as thoseillustrated at 14a in FIG. 1A, even in small amounts, drasticallyreduces the relaxation, retransformation, or tum-off response time inelectro-optic cells and imaging systems such as those described herein.For example, it has been found that the inclusion of the advantageousadditives in some embodiments reduces the relaxation or tum-off responsetime by orders of magnitude up to about 10.

The additives suitable for use in the advantageous system of the presentinvention are believed to typically fall into two general groups: (1)electrophoretic-like (specifically including both electrophoretic anddielectrophoretic additives) centers which are capable of moving withinthe electro-optic liquid crystalline composition when electrical fieldsare placed across the thickness of the compositions; and (2) dipolarparticles which re-align themselves and may move within theelectro-optic liquid crystalline composition in response to theapplication of the electrical fields used in the advantageous system ofthe present invention. In various include dispersed particulate carbon;

ill.--

embodiments hereof, the advantageous additives may be substantiallyinsoluble and/or immiscible, or sub stantially soluble and/or miscible,or combinations thereof. The terms insoluble and immiscible herein referto the substantial insolubility or substantial immiscibility (inabilityto mix in a single homogeneous phase) of the additives with theelectro-optic liquid crystalline composition. Materials suitable for useas the electrophoretic-type additives are typically particles ordispersed globules of substantially electrically insulating materialswhich become triboelectrically charged in the liquid crystallinecomposition-additive mixture. Insoluble or immiscible additives of thistype polyvinyl chloride; polystyrene; Teflon, tetrafluoroethylene resinsavailable from DuPont; silicon carbide; titanium dioxide; silica;sulfur, ammonium chloride; lead chromate; and various other insolublesalts and pigments; polymeric additives such as a copolymer of styreneand n-butyl methacrylate, and even copolymer of styrene and n-butylmethacrylate pigmented with carbon black, and various mixtures andcombinationsthereof. Such materials-are typically used in concentrationswhich are sufficiently low to allow good suspension or emulsification,without significant conglomeration of the additive material. Suchmaterials are preferably used in concentrations of not greater thanabout 15 percent. Particles or globules of such materials are typicallyof average particle size not greater than about 5 microns, and particlesof average size not greater than bout 1 micron give preferred results inthe inventive system.

Where the additives are soluble or miscible, the additive is generallyan organically soluble inorganic salt or organically soluble organiccompound. Such soluble or miscible additives include alkyl ammoniumhalides such as tetraheptyl-ammonium iodide andhexadecyltrimethylammonium bromide; alkyl phosphonium compoundsincluding alkyl phosphonium sulfates, alkyl phosphonium bisulfides,alkyl phosphonium selenides, and alkyl phosphonium phosphates; alkylsulphonium compounds also including alkyl sulphonium sulfates, alkylsulphonium bisulfides, alkyl sulphonium selenides and alkyl sulphoniumphosphates; anils such as pmethoxy-benzylidene-p'-n-butylaniline as wellas mixtures thereof and others.

Materials typically suitable for use as dipolar-type additives includevarious dispersed oils such as cottonseed oil, castor oil, siliconeoils, linseed oil, mineral oil, polysulphone and others. Other suchmaterials include acetonitrile, diethylamine, iodine, ethyl dichloride,amyl acetate, butyl acetate and other alkyl acetates; diethyl ether,monochlorobenzene, toluene, metaxylene, anisole, n-propyl chloride,chloroform, steric acid, xylol, carnauba wax, nitrobenzene, polyvinylchloride, ammonium chloride, mixtures thereof and others. In someembodiments, the above materials may also behave as electrophoretic-likematerials, as described above.

The advantageous additives of the present invention are typically addedin amounts not greater than about 15 percent, and even smaller amountstypically achieve the advantageous results of the present invention.Where the additive is in distinct particulate form, particles of averagesize not greater than about 5 microns are preferred to reduce off-timesof the cells described herein, and particles of average size not greaterthan about 1 micron give particularly preferred or optimum off-times.Although in most embodiments hereof the shape of the additive particlesis not a problem, where dipolar materials are used, elongate moleculesor parti cles are preferred because their rotation apparently disruptsthe nematic alignment to a greater extent than more spherical particles.

During operation of the electro-optic cells of the present inventionhaving an active element comprising a liquid crystalline composition andthe aforementioned advantageous additives, the additive particles areobserved moving through the layer of liquid crystalline compositioneither toward one of the electrodes, or oscillating back and forthbetween the electrodes, where the electrophoretic-type additives areused, or realigning within the liquid crystalline composition matrix,where the dipolar type additives are used. Although it is not clear whythe addition of such additives, and particularly, small amounts of suchadditives, should, in theory, significantly reduce the tum-off responsetime of such electro-optic cells and imaging systems, certain hypotheseshave been made about the operation of this electro-optic liquidcrystalline system. For example, it is believed that when theappropriate threshold field is applied across the layer of electro opticliquid crystalline composition that the presence of the additivesslightly disrupts the alignment of the field-induced nematic liquidcrystalline state to which the transformed portions of the compositionare driven, and that the potential energy inherent in the field-inducedsystem, said system including the advantageous additives of the presentinvention, is greater than the potential energy state of thetheoretically more perfectly aligned field-induced nematic state in asystem which does not contain the advantageous additives. However, eventhis theoretical model of the operation and effect of the presence ofthe advantageous additive particles in the advantageous system of thepresent does not predict the surprising and unexpected decrease in thetum-off response time by several orders of magnitude, as is found in thepractice of some embodiments of the inventive system.

Although methods for providing electrical fields across a layer of theelectro-optic compositions preferred for use in the advantageous systemof the present invention have been described herein with reference toparallel plate electrode cell systems, it is clear that any suitablemeans for providing electrical fields of field strength about or greaterthan the transition threshold strengths in the advantageous system ofthe present invention are suitable for use in various embodiments of theinvention system. For example, any of the address means suggested incopending application Ser. No. 821,565 now US. Pat. No. 3,652,148 may beused, including electron beam address systems, and electron beam addresssystems using other included electrical field generating systems such aspin tubes, or layers of secondary emission materials; X-Y grid addresssystems; electrostatic latent images, for example, electrostatic latentimages on any sort of insulating support which are brought into closeproximity with layers of the electro-optic compositions of the presentinvention, such as an electrostatic latent image on a photoconductorlayer; combined electrical and thermal address systems; and a variety ofmultiple cell, and coplanar multiple cell address systems, as well asany other suitable means for providing the appropriate electrical fieldsacross the electro-optic composition layer. It is again-noted that invarious embodiments the electrical fields may be provided by A.C., orDC. potential sources or any suitable combinations thereof.

The following examples further specifically define the present inventionwith respect to liquid crystalline electro-optic cells and imagingsystems and compositions including electrophoretic-like, or dipolaradditives in such compositions which are particularly suitable tocontrol and minimize the tum-off response time in such systems. Theparts and percentages are by weight unless otherwise indicated. Theexamples below are intended to illustrate various preferred embodimentsof the novel liquid crystalline electro-optic system.

EXAMPLE I A cholesteric liquid crystalline composition comprising about35 percent cholesteryl chloride and about 65 percent cholesterylnonanoate is prepared, and an about 50-50 percent mixture of PbCrO and acopolymer of styrene and n-butyl methacrylate pigmented with carbonblack, is added in an amount comprising about 5 percent of the totalresultant composition. This composition is mixed to suspend the additivein the liquid crystalline composition, and a layer of the composition isplaced into a cell comprising a pair of substantially transparentelectrodes, the electrode surface being a transparent chromium coatingon a glass substrate, and the electrodes enclose the composition in anabout 1 mil thick Mylar spacer gasket (Mylar is a polyethyleneterephthalate resin available from Du- Pont).

A voltage of about 900 V. DC. is provided across the thickness of thecomposition layer, thereby causing the cholesteric-nematic phasetransition. Microscopic observation reveals that the includedparticulate additives bound back and forth between the electrode plates.Recovery or off-times are measured for the above cell and for a controlcell which does not contain the additive. The present cell exhibitsoff-times which are about one-half the duration of off-times in thecontrol cell.

EXAMPLE II A cholesteric liquid crystalline composition comprising about59 percent cholesteryl chloride, about 39.4 percent cholesterylnonanoate, and about 1.6 percent oleyl cholesteryl carbonate, isprepared, and polysulphone is added and dispersed therein in an amountcomprising about 15 percent of the total resultant composition. A layerof this composition is provided in a cell as in Example I, except thatthe spacer gasket is of a thickness of about one-half micron.

A voltage of about 1,500 V., DC. is provided across the thickness of thecomposition layer, thereby causing the cholesteric-nematic phasetransition to occur. Under a microscope, the dispersed additive isobserved to move vigorously throughout the composition. Recovery oroff-times are measured for the above cell and for a control cell whichdoes not contain the additive. The present cell exhibits off-times whichare about one-third the duration of off-times in the control cell.

all. EXAMPLE III A cholesteric liquid crystalline composition comprisingabout 60 percent cholesteryl chloride and about 40 percent cholesterylnonanoate, is prepared, and cottonseed oil is added and dispersedtherein in an amount comprising about percent of the total resultantcomposition. A layer of this composition is provided in a cell as inExample II.

A voltage of about 100 V., DC. is provided across the thickness of thecomposition layer, thereby causing the cholesteric-nematic phasetransition to occur. Recovery or off-times are measured for the abovecell and for a control cell which does not contain the additive. Thepresent cell exhibits off-times which are about one-fourth the durationof off-times in the control cell.

EXAMPLE IV A cholesteric liquid crystalline composition as in ExampleIII is prepared, and castor oil is added and dispersed therein in anamount comprising about percent of the total resultant composition. Acell as in Example I, except that the spacer-gasket is of a thickness ofabout 2 microns.

A voltage of about 1,000 V., DC is applied across the thickness of thecomposition layer, thereby causing the cholesteric-nematic phasetransition to occur. Recovery or off-times are measured for the abovecell and for a control cell which does not contain the additive. Thepresent cell exhibits off-times which are about one-twentieth theduration of off-times in the control cell.

EXAMPLE V A composition as in Example IV is prepared including thecastor oil additive in an amount comprising about 10 percent of thetotal resultant composition. The cell of Example IV is used.

A voltage of about 500 V., DC. is applied across the thickness of thecomposition layer, thereby causing the cholesteric-nematic phasetransition to occur. Recovery or off-times are measured for the abovecell and for a control cell which does not contain the additive. Thepresent cell exhibits off-times which are about one-twentieth theduration of off-times in the control cell.

EXAMPLE VI A cholesteric liquid crystalline composition as in ExampleIII is prepared, and tetraheptyl ammonium iodide is added in an amountcomprising about 0.4 percent of the total resultant composition. A layerof this composition is provided in a cell as in Example I.

A voltage of about 880 V., AC. lOI-Iz, is provided across the thicknessof the composition layer, thereby causing the cholesteric-nematic phasetransition to occur. This all exhibits a response or off-time of about0.5 seconds.

A control cell which does not contain the additive is transformed with avoltage of about 792 V., AC. IOI-Iz, and exhibits a response or off-timeof about 3.8 seconds.

EXAMPLE VII A cholesteric liquid crystalline composition as in ExampleIII is prepared and p-methoxy-benzylidene-p'-nbutylaniline (hereafterMBBA) is added in an amount comprising about 1.0 percent of the totalresultant composition. A layer of this composition is provided in a cellas in Example I.

A voltage of about 500 V., DC. is provided across the thickness of thecomposition layer thereby causing the cholesteric-nematic phasetransition to occur. This cell exhibits a response or off-time of about2.0 seconds. A control cell which does not contain the additive,exhibits a response or off-time of about 20 seconds.

EXAMPLE VIII A composition as in Example VIII is prepared including theMBBA additive in an amount comprising about 9.2 percent of the totalresultant composition. The cell is transformed as in Example VII andexhibits a response or off-time of about 20 milliseconds. A control cellwhich does not contain the additive exhibits a response or off-time ofabout 20 seconds.

The above examples illustrate various cholesteric liquid crystallinecompositions and additives which exhibit the inventive effect of thepresent invention. However, these examples are in no way limiting.Various other liquid crystalline compositions, such as all thosedescribed in application Ser. No. 821,565, now US. Pat. No. 3,652,148are suitable for use herein, and vari ous other additives as disclosedherein provide the advantageous results of the present invention.

Although specific components and proportions have been stated in theabove description of preferred embodiments of the advantageous liquidcrystalline electro-optic. system and novel compositions for usetherein, other suitable materials and variations in the various steps inthe system as listed herein may be used with satisfactory results andvarious degrees of quality. In addition, other materials and steps maybe added to those used herein and variations may be made in the processto synergize, enhance or otherwise modify the properties of or the usesfor the present invention. For example, various other mixtures of liquidcrystals which will undergo the phase-transition imaging process may bediscovered and used in the system of the present invention and suchmixtures may require somewhat different thicknesses, threshold fields,temperature ranges, and other conditions for preferred results inaccordance with the present invention. Likewise, various other means ofcreating electrical fields of the requisite threshold field strength andother means of addressing the inventive electro-optic systems may beused with satisfactory results in the present invention.

It will be understood that various changes in the details, materials,steps, and arrangements of elements which have been herein described andillustrated in order to explain the nature of the invention, will occurto and may be made by those skilled in the art upon a reading of thisdisclosure, and such changes are intended to be within the principle andscope of this invention.

What is claimed is:

1. A method of transforming an optically negative liquid crystallinematerial to an optically positive state comprising providing anoptically negative liquid crystalline composition at a temperature inthe optically negative-optically positive transition range of saidliquid crystalline composition, said composition additionally comprisingan additive of electrophoretic-like or dipolar material dispersedthroughout said liquid crystalline material, and

applying an electrical field across said liquid crystal line materialwithin the optically negative-optically positive transition electricalfield range of said liquid crystalline composition.

2. The method of claim 1 wherein the optically negative liquidcrystalline composition exhibits cholesteric liquid crystallinecharacteristics.

3. The method of producing a cholesteric to nematic phase transition ina liquid crystalline material comprismg providing a cholesteric liquidcrystalline composition at a temperature in the cholesteric mesophasetemperature range of said liquidcrystalline composition, said liquidcrystalline composition additionally comprising an additive ofelectrophoreticlike or dipolar material dispersed throughout said liquidcrystalline composition, and

applying an electrical field across said liquid crystalline compositionwithin the cholesteric-nematic phase transition electrical field rangeof said liquid-crystalline composition.

4. The method of producing a transient electro-optic effect comprisingperforming the method of claim 2 and reducing the electrical fieldacross the liquid crystalline composition to a field strength below thephase transition threshold field strength of the liquid crystallinecomposition.

5. The method of claim 4 wherein the additive is soluble or miscible inthe cholesteric liquid crystalline composition.

6. The method of claim 4 wherein the additive is insoluble andimmiscible in the cholesteric liquid crystalline composition.

7. The method of claim 4 wherein said additive comprises not greaterthan about 15 percent of the entire composition.

8. The method of claim 4 wherein the additive comprises particles ofaverage particle size not greater than about 5 microns.

9. The method of claim 8 wherein the additive comprises particles ofaverage particle size not greater than about 1 micron.

10. The method of claim 4 wherein the liquid crystalline composition isarranged in a layer of said compmi bst t'all t 1 t. i? Th llllfit o t c5311,14 wherein said layer of cholesteric liquid crystalline compositionis shaped in a desired image configuration.

17. The method of claim 14 wherein at least oneof the electrodes isshaped in a desired image configuration.

18. The method of claim 10 wherein the electrical field applied acrossthe liquid crystalline composition is of a field strength not less thanabout 10 volts/cm. of thickness of the liquid crystalline composition.

19'. The method of claim Ill wherein the composition layer is viewedwith means for enhancing contrast between transformed and nontransformedareas thereof.

20. The method of claim 10 wherein the composition layer is betweenpolarizers and viewed with transmitted light.

2. The method of claim 1 wherein the optically negative liquidcrystalline composition exhibits cholesteric liquid crystallinecharacteristics.
 3. The method of producing a cholesteric to nematicphase transition in a liquid crystalline material comprising providing acholesteric liquid crystalline composition at a temperature in thecholesteric mesophase temperature range of said liquid crystallinecomposition, said liquid crystalline composition additionally comprisingan additive of electrophoretic-like or dipolar material dispersedthroughout said liquid crystalline composition, and applying anelectrical field across said liquid crystalline composition within thecholesteric-nematic phase transition electrical field range of saidliquid-crystalline composition.
 4. The method of producing a transientelectro-optic effect comprising performing the method of claim 2 andreducing the electrical field across the liquid crystalline compositionto a field strength below the phase transition threshold field strengthof the liquid crystalline composition.
 5. The method of claim 4 whereinthe additive is soluble or miscible in the cholesteric liquidcrystalline composition.
 6. The method of claim 4 wherein the additiveis insoluble and immiscible in the cholesteric liquid crystallinecomposition.
 7. The method of claim 4 wherein said additive comprisesnot greater than about 15 percent of the entire composition.
 8. Themethod of claim 4 wherein the additive comprises particles of averageparticle size not greater than about 5 microns.
 9. The method of claim 8wherein the additive comprises particles of average particle size notgreater than about 1 micron.
 10. The method of claim 4 wherein theliquid crystalline composition is arranged in a layer of saidcomposition.
 11. The method of claim 10 wherein said layer is of athickness not greater than about 250 microns.
 12. The method of claim 11wherein said layer is of a thickness in the range between about 1 micronand about 50 microns.
 13. The method of claim 4 wherein the electricalfield placed across the composition is in an imagewise configuration.14. The method of claim 10 wherein the layer of composition is providedbetween a pair of electrodes, one of which is substantially transparent.15. The method of claim 14 wherein both electrodes are substantiallytransparent.
 16. The method of claim 14 wherein said layer ofcholesteric liquid crystalline composition is shaped in a desired imageconfiguration.
 17. The method of claim 14 wherein at least one of theelectrodes is shaped in a desired image configuration.
 18. The method ofclaim 10 wherein the electrical field applied across the liquidcrystalline composition is of a field strength not less than about 102volts/cm. of thickness of the liquid crystalline composition.
 19. Themethod of claim 10 wherein the composition layer is viewed with meansfor enhancing contrast between transformed and nontransformed areasthereof.
 20. The method of claim 10 wherein the composition layer isbetween polarizers and viewed with transmitted light.